In the fields of electrical appliances, automobiles, etc., importance of the products placing emphasis on design esthetics is increasing recently. The material used therefor is also required to have high mechanical performance, light weight and good appearance to conform to such a trend. As an answer to such a request, in the field of materials where transparency is required, there have been developed and used polycarbonate resins belonging to the field of material called engineering plastics, and SMI resins which are copolymers of styrene and N-phenylmaleimide. These resins have excellent heat resistance, but they are inferior to the conventional AS resins in moldability, and are also unsatisfactory in terms of price. It is especially remarkable that the recent resin products have complicate configurations to meet the esthetic request and are also increasingly thin-sectioned for the lightweight and compact design, so that ease of handling in molding and processing is attracting attention as a part of performance of the material from the aspects of improvement of yield, energy saving and resources saving. From such a viewpoint, pursuit of a novel resin material that can meet both requirements for easy handling like AS resins which have been utilized in the wide fields of industries and for high heat resistance like engineering plastics has been made.
Further, to answer the above request, modified polyphenylene ethers and heat resistance-improved version of ABS resins, or so-called heat-resistant ABS resins, have been developed and used. Particularly heat-resistant ABS resins, for which various methods of improvement have been developed, are advantageous in moldability, weather resistance and cost over other engineering plastics and widely used as heat resistant resin material. In order to improve heat resistance of AS resin moiety in ABS resin, there is generally employed a so-called graft blending method in which an acrylonitrile-styrene-.alpha.-methylstyrene terpolymer obtained by polymerizing acrylonitrile, styrene with .alpha.-methylstyrene or an acrylonitrile-styrene-.alpha.-methylstyrene-N-phenylmaleimide quadripolymer is blended with a rubber-reinforced resin to form a resin composition. In this case, since containing .alpha.-methylstyrene is an essential factor for the improvement of heat resistance, there arises the problem that when the content of this monomer is small, no satisfactory heat resistance can be obtained, and when its content is large, since the polymerization rate is lowered, it is difficult to obtain a resin with high degree of polymerization, and there is produced a chain structure which tends to cause thermal decomposition during processing. A heat-resistant ABS resin using a maleimide-based copolymer for the purpose of eliminating said disadvantage caused by use of .alpha.-methylstyrene has been developed (JP-A-61-16955, etc.).
Also, resin compositions using specific maleimide-based copolymers for providing maleimide-based copolymer resin compositions with excellent impact resistance are disclosed in JP-A-2-51514 and JP-A-2-196849.
However, maleimide-based copolymers, although high in heat resistance, have the disadvantage in that they are poor in molding processability as they are low in fluidity in the molten state as compared with conventional AS resins. For improving molding process-ability, usually a plasticizer, a lubricant, etc., are added, but this gives rise to the problems such as the necessity of uniformalizing dispersion of the additives in the resin, and exudation of the additives onto the surface of the molded product during the molding and processing to spoil appearance of the molded product or reduce its heat resistance contrary to the original object.
The resin used in the field of exterior parts of vehicles such as lamp housing of automobile needs to be a thermoplastic resin which has excellent heat and weather resistance in addition to impact resistance. A typical example of impact-resistant thermoplastic resin is ABS resin.
Recently, for the purpose of enhancing heat resistance of ABS resin, there is used a maleimide-based copolymer produced by copolymerizing a maleimide compound, used as matrix resin, with an unsaturated cyanogen compound and an aromatic vinyl compound. To combat poor processability of heat-resistant ABS resin incorporated with said maleimide-based copolymer, it has been proposed to use a maleimide-based copolymer containing an oligomer such as disclosed in Japanese Patent Application No. 6-989.
On the other hand, ABS resin has the defect that it is poor in weather resistance because of use of polybutadiene, which is a conjugated diene rubber and susceptible to decomposition by ultraviolet ray, as rubber component. For improving the weather resistance, AAS resin using an acrylic ester rubber as rubber component is used, but AAS resin is inferior to ABS resin in impact resistance. For the purpose of improving impact resistance of AAS resin, use of a rubber obtained by compounding a minor amount of a conjugated diene rubber and a major amount of an acrylic ester rubber, such as disclosed in JP-B-3-66329, has been proposed to make a specific AAS resin.
Therefore, as means for obtaining a thermoplastic resin having excellent heat and weather resistance, it appears effective to blend a maleimide-based copolymer and the specific AAS resin.
However, the thermoplastic resin composition obtained by blending a maleimide-based copolymer and the specific AAS resin, although having heat and weather resistance, has the problem that the surface appearance of the molded product is deteriorated. That is, it has the problem of causing cloudiness or partial disappearance of gloss or nonuniform gloss, referred to comprehensively as defective appearance, of the surface of the molded product.